Belt or conveying tables are typically used to convey printed sheets to a printing press. To enable a close sequence of sheets to be achieved at high speeds, the printed sheets are usually conveyed in overlapping relationship on such tables by conveyor belts. It has been found very advantageous to operate the conveying table by suction. Unfortunately, the facilities previously utilized for this purpose have many disadvantages.
For example, DE-OS 3,838,078 discloses a device for conveying an overlapping stream of sheets wherein the conveying table is in the form of a suction box to which an axial flow fan is connected and perforate suction belts are guided over orifices in the conveying table. When the fan operates, the interior of the table is exhausted and paper sheets moving over the table are sucked onto the suction belts and conveyed onwards thereby. In special instances, the arrangement includes an additional suction chamber at the delivery end of the table, with separate controllability, and the arrangement of the suction belts in guide grooves below the plane of the table, so that the surface of the suction belts is substantially at the height of the plane of the table.
The foregoing system operates generally satisfactorily and reliably for ordinary papers, but problems arise with special papers, more particularly, relatively thin paper stocks. First, the suction belts elongate in operation in the course of time so that an undefined slip occurs between the suction belts and the drive rollers. Such slip can, of course, be countered by take-up devices but the drive ratios cease to be equal and constant in time on both sides of the table. Second, the suction belts do not always lie completely flat on the table or in the guide grooves, so that a relatively large air gap arises along the belt edges. This is unavoidable partly because of the suction belt edges curling up and partly because of surface irregularities in the belt and supporting groove. Consequently, air is sucked in laterally through and adjacent the suction belts and the negative pressure is propagated over large areas of the conveying table. As a result, the paper stock is sucked around the suction belt edges and then grazes against the table severely, often causing late or skewed sheets in the lays. Also, due to edge-curling, the sheets no longer lie flat when they arrive at the lays.
Further problems arise in the case of intermittent drive such as occurs, for example, when the printed sheets entering the lays are retarded cyclically, for example, by the drive of the conveying or suction belts varying in speed periodically at the cadence of the sheets. Since it is precisely in the transition zone between the table and the lays that the sheets are retained on the suction belts only by way of a reduced suction area, it becomes impossible to control sheet position sufficiently at the time of maximum decelerations. The prior art attempts to solve this problem by using an additional suction chamber to exert in the front zone of the table a greater negative pressure on the sheets than in the rear zone. In this arrangement, however, the suction must be timed and the table becomes more complex since it has been found in practice that a separate blower may be necessary to produce the negative pressure in the additional suction chamber. This increased negative pressure is very unsatisfactory particularly in the case of relatively thin papers because of the risk of papers being sucked through with the result of impairment of sheet alignment. Consequently, for both relatively thin papers and relatively thick boards the use of a large number of auxiliary facilities such as pressing rollers, braking brushes, ball riders, elaborate suction air regulation systems or the like are employed to ensure that the sheets are guided on the table with some reliability.